1. Technical Field
This invention relates to methods and apparatus for non-invasively determining biological tissue oxygenation utilizing near-infrared spectroscopy (NIRS) techniques in general, and to sensors for use with such techniques in particular.
2. Background Information
Near-infrared spectroscopy is an optical spectrophotometric method that can be used to continuously monitor tissue oxygenation. The NIRS method is based on the principle that light in the near-infrared range (700 nm to 1,000 nm) can pass easily through skin, bone and other tissues where it encounters hemoglobin located mainly within micro-circulation passages; e.g., capillaries, arterioles, and venuoles. Hemoglobin exposed to light in the near- infrared range has specific absorption spectra that varies depending on its oxidation state; i.e., oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) each act as a distinct chromophore. By using light sources that transmit near-infrared light at specific different wavelengths, and measuring changes in transmitted or reflected light attenuation, concentration changes of the oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb) can be monitored. The ability to continually monitor cerebral oxygenation levels, for example, is particularly valuable for those patients subject to a condition in which oxygenation levels in the brain may be compromised, leading to brain damage or death.
NIRS type sensors typically include at least one light source and one or more light detectors for detecting reflected or transmitted light. The light signal is created and sensed in cooperation with a NIRS system that includes a processor and an algorithm for processing signals and the data contained therein. U.S. Pat. No. 7,047,054 and PCT Published Application No. WO 07/048039, which are commonly assigned with the present application to CAS Medical Systems, Inc. of Branford, CT, disclose examples of such a sensor. Light sources such as light emitting diodes (LEDs) or laser diodes that produce light emissions in the wavelength range of 700-1000 nm are typically used. A photodiode or other light detector is used to detect light reflected from or passed through the tissue being examined. The NIRS System cooperates with the light source(s) and the light detectors to create, detect and analyze the signals in terms of their intensity and wave properties. U.S. Pat. Nos. 7,047,054 and 7,072,701, which are commonly assigned to CAS Medical Systems, Inc., of Branford, CT, disclose a methodology for analyzing such signals. U.S. Pat. No. 7,072,701, and PCT Published Application No. WO 07/048039are hereby incorporated by reference in their entirety.
Meaningful cerebral oxygenation information is collected from light interrogating brain tissue (e.g., passing through, reflecting from, absorbed by, etc.). To non-invasively access the brain tissue, however, the light signal must pass through extracerebral tissue (e.g., scalp, skull, etc.) before and after interrogating the brain tissue. A light signal traveling within any biological medium (e.g., tissue, fluid, etc.) will attenuate, and the amount of attenuation is a function of the medium. In the case of a mean optical path that non-invasively accesses brain tissue, the attenuation attributable to the extracerebral tissue does not yield useful information with respect to the cerebral oxygenation. Consequently, it is desirable to account for the signal attenuation attributable to extracerebral tissue, so that the attenuation attributable to the brain tissue can be distinguished and analyzed.
It is known to use a NIRS sensor having a pair of light detectors specifically spaced apart from a light source as a means to account for extracerebral tissue. A “near” light detector may be spaced apart from a light source by a first separation distance, and a “far” detector may be spaced apart from the light source by a second separation distance, which is typically greater than first separation distance. The method for spectrophotometric blood oxygenation monitoring disclosed within U.S. Pat. No. 7,072,701 is an example of a method that can be used with two detectors.
A problem common to all NIRS sensors is signal interference from ambient light sources, electromagnetic interference (EMI) sources, etc. Mitigating the effect of such interference improves the quality of the signal available, and therefore the patient information available. Another problem common to all NIRS sensors is the cost to manufacture. NIRS sensors are typically disposed of after use, so the cost of the sensor is an important factor in the cost of the monitoring.
What is needed, therefore, is an improved sensor for non-invasively determining the level of oxygen saturation within biological tissue, one that can be configured with one or more detectors, one that mitigates interference, and one that can be readily manufactured.